Tunable magnetrons



y 1956 E. c. DENCH ETAL 2,745,987

TUNABLE MAGNETRONS 3 Sheets-Sheet l Filed Jan. 17, 1952 f @U 4 4 E H- a 0 2 .45 5 ,2 I Wm E u K C \1 3 m May 15, 1956 E. c. DENCH EI'AL 2,745,987

TUNABLE MAGNETRONS Filed Jan. 17, 1952 3 Sheets-Sheet 2 aygg l y 1956 E. c. DENCH HAL 2,745,987

TUNABLE MAGNETRONS 3 Sheets-Sheet 3 Filed Jan. 1'7, 1952 PENETRATION DEPTH IN INCHES www n. w% N M [0M 7 M w m /M0 M y EM 5 m United States Patent @flice 2,745,987 Patented May 15, 1956 TUNABLE MAGNETRONS Edward C. Bench, Necdham, Mass, and George E. Dombrowski, Great Necir, N. Y., assignors to Raytheon Manufacturing Company, Newton, Mass, :1 corporation of Delaware Application January 17, 1952, Serial No. 266,912

13 Claims. (Cl. 315-3961) This invention relates to electron discharge devices of the magnetron type and, more particularly, to apparatus for modulating the frequency of magnctrons.

it is well known that cavity magnetrons are an eificient source of high frequency or microwave oscillations. However, great dificulty has been experienced in efliciently modulating the frequency of cavity magnetrons. Cavity magnetrons have been electrically tuned by varying the density of electrons in the cavity, thereby varying the dielectric constant between the walls of the cavity, and hence tuning the cavity. While such devices may be frequency modulated with relatively high modulation frequencies, they are, as a rule, less efiicient than devices which are mechanically tuned.

Mechanically-tuned magnetrons have been previously made in two manners. The first manner is by tuning a cavity or length of wave guide which is coupled to the magnetron through a section of wave guide. Such devices vary the impedance match between the magnetron and the wave guide or other transmission line which connects the magnetron to its load, and as a result the power output of the magnetron is varied with the frequency. This is, obviously, not desirable since it contributes to distortion of the frequency-modulated signal and decreases the eificiency of the system for frequency modulation purposes. The second system for mechanically tuning a magnetron utilizes insertion of a tuning element directly into a cavity or cavities of the magnetron. The element is mechanically moved by means of an actuator which is connected to the outside element through a bellows or diaphragm arrangement. Such a device, while satisfactory for adjusting the operating frequency of the magnetron, is inadequate for modulation purposes since the bellows is rapidly fatigued by movement thereof and will fail after a short period of continuous modulation.

This invention discloses a mechanical tuning arrangement which requires no bellows and yet is closely coupled to the magnetron-anode structure since the tuning element is inserted into a portion of one of the cavities of the anode structure. Briefly, this is accomplished by placing a ceramic cup in an aperture extending from the outside of the magnetron into one of the cavities in the anode structure. The tuning element is then placed inside the cup and moved with respect to the anode structure, thereby varying the resonant frequency of the cavity, and hence tuning the magnetron. Due to the closeness of coupling of the tuning element to the anode structure, the tuning of the magnetron frequency may be made a relatively linear function of the displacement of the tuner element over a relatively wide range of frequencies. Furthermore, since there is no diaphragm attached to the tuning element, the apparatus will have a long life, and the tuning element itself will require a relatively low modulating power since no energy will be dissipated through a diaphragm.

Other and further objects and advantages of the invention will become apparent as the description thereof progresses, reference being had to the accompanying drawings wherein:

Figs. 1 and 2 illustrate longitudinal cross-sectional views of a magnetron discharge device embodying this invention taken along lines 11 and 2-2 of Figs. 2 and 1, respectively;

Fig. 3 illustrates a detail of the cathode-mounting structure; and

Fig. 4 illustrates a graph showing the relationship of output frequency to movement of the tuning element.

Referring now to Figs. 1, 2, and 3, there is shown an anode block 10 having a cylindrical hole 11 therein. Positioned in the hole 11 is an anode structure comprising an annular ring 12. Extending radially inwardly from the inner surface of annular ring 12 is a plurality of anode members 13 comprising substantially rectangular planar members Whose planar surfaces lie substantially parallel to the axis of hole 11. Anode members 13 are alternately connected at points adjacent their inner ends on the upper and lower edges thereof by conductive straps 14 which prevent oscillations of the device at undesired spurious mode frequencies.

Positioned inside the space defined by the inner ends of anode members 13 is a cathode 14a comprising a cylindrical bar of sintered thoria, the axis of which is coaxial with the hole 11 and the anode ring 12. The ends of cathode 14-12 are attached to metal cylinders 15, said cylinders extending over the ends of cathode 14a to a point slightly beyond the upper and lower edges of the anode members 13, thereby acting as end shields. Cylinders 15 are rigidly attached to the cathode 14a by sintering, soldering, or any other desired method. Cylinders 15 are attached as by brazing to support discs 16 which surround cylinders 15, discs 16 being attached by means of ceramic screws 17 to ceramic cylinders 18 which extend into recesses 19 in anode ring 12, and are threadedly attached to metallic studs 29 therein. Ceramic cylinders 18 are locked on to studs 2! by a solder ring 21 surrounding the ceramic rings 13 in the recesses 19. The ring 21 is flowed into the recesses 19 in melted form and upon cooiing contracts, thereby compressing the ceramic cylinder 18 so that the thread portion thereof engaging the studs 29 cannot unscrew. Discs 16 have radial slots 22 cut therein from their outer edge to a point somewhat near the center of the discs. The discs 16 also have an annular ridge 23 therein. The purpose of the annular ridges 23 and the slots 22 is to prevent radial stresses on the ceramic bushings 18 due to expansion and contraction of the discs 16 upon heating while still allowing the discs 16 to have a substantial radiating surface. As a result, the cathode 14 may be maintained at a sufficiently low temperature to prevent overheating and burning even when the device is operated at extremely high power levels with the attendant high heating of the cathode due to back bombardment of electrons and positive ions.

The cathode cylinders 15 extend outwardly through cylindrical holes in pole pieces 24 which are set at apertures in end plates 25 covering the ends of the hole 11 in the anode block it? and are connected through flexible conductors 15a to lead-in conductors 2652. A vacuum seal is produced between lead-in conductors 26a and the pole pieces 24 by means of glass seals 26 which are only partially shown, but may be of any well-known type. Thus, the cathode 14a is insulatedly supported with respect to the anode structure, and may be heated to electron-emitting temperature by applying a heater voltage between the portions of the support cylinders 15 that extend out beyond the glass seals 26. After operation of the device starts, the heater voltage may be substantially reduced or even eliminated, and the cathode 14 will remain hot due to back bombardment.

An output coupling is provided for the magnetron comprising a slot 27 extending from one of the cavities, defined by a pair of anode members 13 and the space therebetween, through the ring 12 and the anode block 10. A cylindrical hole 28 extends through the anode block surrounding the slot 27 and fillet blocks 29 are positioned in hole 28 such that slot 27 will have a substantially uniform thickness and width as it passes through anode ring 11 and anode block It Such an output coupling device is known as an H slot coupler since a cross-sectional view through the slot as it passes through the anode block has a modified H shape.

The H slot output passes through a plate 39' attached to anode block It surrounding the H slot output. A metallic cup 31 has the lip thereof attached to plate 39 by being soldered to a projection on plate 39. The bottom of cup 31 has the aperture therein substantially the same size as the cylindrical hole 28 and is coaxial therewith. A transparent seal 32 is positioned in the aperture in cup 31 and produces a vacuum seal therewith. The output coupling device is adapted to feed energy through window 32 into a circular wave guide, partially shown at 33 as being threadedly attached to plate 39 and surrounding the window 32. The dimensions of the cup 31, window 32, and the H 'slot output coupling are so designed as to produce an optimum match from the magnetron to the wave guide 33. An evacuation tube 34 is rigidly attached to an aperture 35 in anode block 10 communicating with the interior thereof, tube 34 after evacuation of the device being pinched, as at 36, to produce a seal.

Provision is made to cool the anode block 10 by means of fluid by placing on the sides thereof, not occupied by the output coupling, the tuning structure or the cathode lead-in members, plates 59 containing passages 60 through which a fluid such as water is adapted to flow. The passages 66 are connected at one end through anode block 1% by means of a hole 61 such that water may be made to flow into the passage in one of the plates 59 through the hole 61 in the anode block 10 and out.

through the passage in the other plate 59.

There is provided a structure 37 for mechanically tuning the anode structure of the device as follows. A hole is cut in the anode block 10 and annular ring 12, said hole extending to a point somewhat short of the inner surface of annular ring 12, and being coaxial with the output coupling hole 23 and on the opposite side of the anode block therefrom. A slot 33 is cut through the remainder of the anode ring 12 which separates the interior of the anode structure from the hole occupied by the tuning structure. The inner portion 12a of ring 12 is somewhat thinner than the outer portion thereof, and the hole extending therethrough from anode block it) and the outer portion of ring 12 has been shaped to form a slot as at 38a which connects slot 38 with the remainder of said hole. Positioned in said hole, whose diameter may be, for example, on the order of a quarter wave length of the operating frequency of the device, is a ceramic cup 39, the bottom of which extends into slot 38a and is positioned adjacent slot 38. Ceramic cup 329 has the lip thereof bonded to a metallic ring 40, which may be made, for example, of Kovar, and which extends outwardly through anode block 10. At the outer edge of anode block 10, the Kovar cylinder 40 is rigidly attached as by soldering through a metallic ring 41 to anode block 11?. An annular recess 42. is provided in anode block 10 surrounding the Kovar cylinder 40, at its point of attachment to ring 41, to provide for relief of stresses set up in this area during the cooling period of the soldering operation. The Kovar ring it? and ceramic cup 39 are spaced slightly from the anode block at all places except the point of attachment of cylinder 40 to the ring 41, thereby insuring that variations in the dimensions of the hole in the anode block surrounding these members will not damage them, for example, by crushing.

A tuning element 43 in the form of a rod is shown partially inserted into the ceramic cup 39. A modulating support 44 is shown which produces movement of rod 43 axially of cup 39 in response to a modulation signal applied thereto but prevents movement of rod i3 in a direction perpendicular to the axis of cup 39. Support 4-4 may be of any desired type and is illustrated herein by way of example only. Support 44 comprises a plate 45 rigidly attached, for example, by screw 46 to anode block 18 surrounding rod 43. An aperture 47 is provided in plate 4-5 to allow the passage of rod 43 therethrough spaced therefrom. Attached to plate 45 by means of screws 48 is a support casing member 49 which comprises the lower end plate of the modulating unit of support 44-. Attached to plate 49 by means of rivets 50 is a spider 51, similar to the spider supporting the voice coil of a conventional loud speaker. The spider 51 is attached at its center to a cup-shaped member 52 on which is wound an actuating coil 53. Rigidly attached to cup 52, and extending through the center thereof coaxial therewith, is a cylinder 54 which surrounds'the tuning element rod 43. The rod 43 threadedly engages cylinder 54 at the upper end thereof such that by rotation of rod 43 in cylinder 54 the mean penetration depth of rod 43 into ceramic cup 39 may be adjusted. A unidirectional magnetic field in the region of the actuating coil 53 is provided by means of a cylindrical member 55 of magnetic material surrounding actuating coil 53, and a cylindrical member 56 extending inside cup 52 surrounding cylinder 54 and engaging cylinder 55 at a point above the lip of cup 52. Members 55 and 56 are magnetized such that a magnetic field appears between the lower ends of members 55 and 56. A casing member 57 is shown surrounding cylinder 55, and screws 58 are shown extending from lower end plate 49 upwardly through member 56 where they are adapted to engage an upper casing end plate, not shown, to which may be attached an upper support spider, not shown, but which may be similar to spider 51, for supporting the upper end of cylinder 54 with respect to the case of the support structure.

Upon the application of suitable heater voltage between cathode supports, anode-cathode potential and a suitable magnetic field in the space between the ends of the anode members and the cathode, in accordance with well-known practice, the device may be made to oscillate at the 11' mode frequency wherein each of the cavities, defined by the anode members 13, is resonant. The cavity defined by the members 13, into which slot 38 enters, has as a portion thereof the area below the end of the tuning member 43 in the hole and the slot 38a in which the ceramic cup 39 is inserted. The size of this cavity is varied by varying the penetration depth of rod 43, thereby varying the resonant frequency of this cavity. Since this cavity has dimensions considerably larger than the other cavities of the anode structure, it will resonate at a lower frequency than the other cavities. Accordingly, since all the cavities of the anode structure are closely coupled together, the reactance presented by this cavity will alter the frequency of the other cavities, thereby altering the oscillating frequency of the device. The amount by which the reactance of the enlarged cavity alters the operating frequency of the device will, therefore, vary as a function of the reactance which, in turn, varies as a function of the penetration of the rod 43 into the ceramic cup 39.

Referring now to Fig. 4, there is shown a graph illustrating operation of device shown in Figs. 1 through 3. Plotted along the axis of abscissae is penetration depth in inches of the rod 43 from the surface of the anode block surrounding the Kovar cylinder 40. Plotted along the axis of ordinates is the change in frequency in megacycles. At the penetration depth of .270 inch, the zero change or reference frequency for a particular device, built in accordance with this invention, was 9832.5 megacycles and is illustrated at point 61 on the graph. As the penetration depth is increased, for example, to .300 inch, the frequency increases by approximately 4.5 megacycles, as is indicated by point 62 on the graph. This increase is in a somewhat non-linear fashion. As the penetration is increased to approximately .350 inch, the frequency increases in a substantially linear fashion to a value of approximately 20 megacycles above zero change frequency, as is indicated by point 63 on the graph. Thus, it may be seen that by this device a linear frequency deviation in excess of megacycles may be achieved in the area between points 62 and 63 on the graph. This linear frequency deviation may be produced by the application of a substantially linearly varying current to the actuating coil 53.

It is to be clearly understood that the particular magnetron structure shown herein is by way of example only, and any type of configuration of anode resonator could be used. Furthermore, the support 44 and the modulation actuating means incorporated therein are shown by way of example only, and any desired tuning actuator could be used.

This completes the description of the particular embodiments of the invention illustrated herein. However, many modifications thereof will be apparent to persons skilled in the art. Accordingly, it is desired that this invention be not limited by the particular details of the embodiment illustrated herein except as defined by the appended claims.

What is claimed is:

l. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, means tightly coupled to one of said cavities in the inductive regions thereof for varying the frequency-responsive characteristic of said one cavity, and a wall substantially transparent to radiant energy positioned between said characteristic-varying means and portions of said cavity.

2. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, means tightly coupled to one of said cavities for varying the frequency-responsive characteristic of said one cavity, and a wall substantially transparent to radiant energy separating said characteristic-varying means from portions of said cavity.

3. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled frequency-responsive members adapted to interact with electrons from said source, means tightly coupled to one of said frequency-responsive members for varying the inductive characteristic of said one member, and a wall substantially transparent to radiant energy separating said characteristic-varying means from said one member, said wall comprising part of the evacuated envelope of said device.

4. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, means tightly coupled to one of said cavities for varying the frequency-responsive characteristic of said one cavity, and a wall substantially transparent to radiant energy separating said characteristic-varying means from portions of said cavity, said wall comprising part of the evacuated envelope of said device.

5. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled frequency-responsive members adapted to interact with electrons from said source, means for producing a magnetic field in the space between said source and said anode structure, means tightly coupled to one of said frequencyresponsive members for varying the frequency-responsive characteristic of said one member, and a wall substantially transparent to radiant energy positioned between said characteristic-varying means and said one member.

6. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled frequency-responsive members adapted to interact with electrons from said source, means for producing a magnetic field in the space between said source and said anode structure substantially perpendicular to the direction of motion of electrons in said space, means tightly coupled to one of said frequency-responsive members for varying the frequency-responsive characteristic of said one member, and a wall substantially transparent to radiant energy positioned between said characteristic-varying means and said one member.

7. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source and a ceramic cup extending into one of said cavities, said cup having positioned therein means for varying the resonant frequency of said one cavity.

8. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source and a ceramic cup extending into one of said cavities, said cup having positioned therein means for varying the resonant frequency of said one cavity, said cup comprising part of the evacuated envelope of said device.

9. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, means for producing a magnetic field in the space between said source and said anode structure, and a ceramic cup extending into one of said cavities, said cup having positioned therein means for varying the resonant frequency of said one cavity, said cup comprising part of the evacuated envelope of said device.

10. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, means for producing a magnetic field in the space between said source and said anode structure, and a cup transparent to radiant energy extending into one of said cavities, said cup having positioned therein means for varying the resonant frequency of said one cavity, said cup comprising part of the evacuated envelope of said device.

ll. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, means for producing a magnetic field in the space between said source and said anode structure substantially perpendicular to the direction of motion of electrons in said space, and a ceramic cup extending into one of said cavities, said cup having positioned therein means for varying the resonant frequency of said one cavity, said cup comprising part of the evacuated envelope of said device.

12. An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, means for producing a magnetic field in the space between said source and said anode structure, a ceramic cup extending into one of said cavities, and means for varying the resonant frequency of said one cavity positioned outside the evacuated enevlope and extending into said cup.

131 An electron discharge device comprising an electron source, an anode structure spaced from said source, said anode structure comprising a plurality of tightly coupled cavities adapted to interact with electrons from said source, a ceramic cup extending into one of said cavities, said cup comprising part of the evacuated envelope of said device, and means for varying the resonant frequency of said one cavity positioned outside the evacuated envelope and extending into said cup.

, References Cited in the file of patent NITED STATES PATENTS Hansen et al May 5, 1942 Hartman Ian. 14, 1947 Spencer L. Feb. 17, 1948 Brown June 21, 1949 Sproull Jan. 3, 1950 

